1. Field of the Invention
The present invention relates generally to methods and apparatus for improving the performance of alternating current (AC) machines and, more specifically, to methods and apparatus for improving the performance of polyphase AC machines through the injection of harmonic frequencies into the excitation current.
2. Description of the Related Art
With conventional polyphase AC machines, both induction and synchronous, the machines are typically operated by a single frequency source. The machines have armature windings to which the single frequency sine waves are applied. The ideal performance of conventional polyphase AC machines includes a sinusoidal air-gap flux wave of constant amplitude rotating around the air-gap at synchronous speed. In this theoretical, ideal polyphase AC machine, the constant amplitude flux wave produces a constant electromagnetic torque. The torque of the machine is monotonically dependent upon this constant amplitude flux wave. This ideal situation is approximated in large scale machines.
In conventional AC machines the magnetic flux per pole of the machine is proportional to the area of 1/2 sine wave of the air-gap flux wave of the machine. Typically, a conventional AC machine is designed to operate with at least one of the magnetic members, the iron core or teeth, of the machine in flux saturation. In conventional machines, the saturation flux densities of the iron, or other magnetic members, of the stator and rotor determine the maximum amplitude of the air-gap flux wave. In conventional machines, therefore, the amplitude of the fundamental flux sine wave determines the maximum power output of the machine. This is true even though maximum use is not made of all of the flux capability of the magnetic members.
In conventional AC machines, undesirable space harmonics are typically established in the air-gap flux waves. These naturally-arising space harmonics occur as a function of the particular machine design when excited by a fundamental frequency. Factors such as slots in the machines and core saturation contribute to the generation of these undesirable space harmonics. These space harmonic flux waves are undesirable because they typically rotate in the air-gap at speeds other than that at which the fundamental flux wave rotates. Additionally, the space harmonic flux waves travel in both forward and backward directions, as well as at different speeds, relative to the fundamental flux wave.
For example, a naturally-arising fifth space harmonic flux wave travels in a reverse direction relative to the fundamental flux wave and at 1/5 the speed of the fundamental flux wave. Similarly, a naturally-arising seventh space harmonic flux wave travels in the same direction as the fundamental flux wave, but at 1/7 the speed. These space harmonic flux waves interact with the squirrel cage winding in an induction motor, or with the damper winding in a synchronous motor, to produce a braking torque that reduces the useful output of the machine. Additionally, these naturally-generated space harmonic flux waves interact with each other, and with the fundamental flux wave, to cause pulsations in the torque of the machine, as well as unwanted mechanical vibrations.
Accordingly, the present invention provides a new method and apparatus for constructing and operating a polyphase AC machine whereby a harmonic flux wave travels in the same direction, and at the same (synchronous) speed, as the fundamental flux wave and whereby the fundamental flux wave is augmented in response to the harmonic flux wave so as to achieve improved electromagnetic loading in the magnetic path of the machine; both achievements serving to improve the useful output of a given machine.